Thermal electromagnetic radiation detector with alveolate structure

ABSTRACT

An absorbent membrane ( 1 ) is fixed in suspension onto a front face of a substrate ( 2 ), in a direction substantially parallel to the substrate ( 2 ), by at least one alveolate structure thermally insulating the membrane from the substrate ( 2 ) and arranged in a plane substantially perpendicular to the substrate ( 2 ). The detector can comprise arms ( 3 ) fixedly secured to the absorbent membrane ( 1 ). The alveolate structures can be respectively arranged between one of the arms ( 3 ) and the substrate ( 2 ). The alveolate structure can be formed by a plurality of superposed thin layers ( 6 ) separated by spacers ( 7 ) or by superposed rows of arcades formed by thin layers. The alveolate structure can comprise a porous pad.

BACKGROUND OF THE INVENTION

The invention relates to a thermal electromagnetic radiation detectorcomprising an absorbent membrane fixed in suspension onto a front faceof a substrate, in a direction substantially parallel to the substrate,by support means thermally insulating the membrane from the substrate.

STATE OF THE ART

Recent technical advances in silicon microelectronics and thin filmproduction have given a new boost to the technology of thermal infraredradiation detectors comprising an absorbent membrane fixed in suspensiononto a substrate by support means.

Moreover, microelectronics is based on collective processes, performedat silicon wafer level, which processes thermal detector technologiescan take advantage of in many respects. These collective techniques infact open up the possibility of achieving detector matrices of largecomplexity, typically matrices of 320×240 detectors, and also ofachieving a large number of matrices on a single silicon wafer andtherefore of reducing the unit production cost of the detectors. Thisfeature, associated with the fact that thermal detectors can operate atambient temperature and not require any cooling system, makes thistechnology particularly well suited for achieving low-cost infraredimagery systems.

FIGS. 1 and 2 show a membrane 1, absorbent with respect to incidentelectromagnetic radiation, of a thermal detector, kept in suspensionsubstantially parallel to a substrate 2 by support means comprising twothermal insulation arms 3, fixedly secured to the membrane 1 andarranged substantially in the plane of the membrane 1. The arms 3 areboth fixed to the substrate 2 by means of pillars 4 dimensioned tosupport the weight of the membrane. Due to the effect of the radiation,the membrane heats and transmits its temperature to a thermometerarranged on the membrane, for example a thermistor in the form of a thinfilm deposited on the membrane.

The substrate 2 can be formed by an electronic circuit integrated on asilicon wafer comprising, on the one hand, the thermometer stimuli andreading devices and, on the other hand, multiplexing components whichenable the signals from different thermometers to be serialized andtransmitted to a reduced number of outputs to be used by a usual imagerysystem. To improve the sensitivity of the thermal detector, the supportmeans are designed in such a way as to insulate the absorbent membrane 1thermally from the substrate 2, thus enabling the heat losses of themembrane to be limited and, consequently, preserving heating of themembrane.

Electrical interconnection between the thermometer and the readingelements arranged on the substrate 2 is generally performed by a metallayer arranged on the support means.

A simplified analysis of heating (Θ) of the membrane due to the effectof the power effectively absorbed (P_(abs)(t)) from the incidentradiation can be performed, a priori, without any particular assumptionon the nature of the thermometer. The heating balance, dependent on thethermal conductance G_(th) of the support means, representing the heatlosses, and on the heat capacity C_(th) of the membrane, representingthe thermal inertia, can be expressed approximately by the followingdifferential equation:

${{C_{th}\frac{\mathbb{d}\Theta}{\mathbb{d}t}} + {G_{th}\Theta}} = {{P_{abs}(t)}.}$This equation finds its particular solution, for a radiation powermodulated in sinusoidal manner at the pulse ω in the expression:

${\Theta = \frac{{\hat{P}}_{abs}(t)}{G_{th}\sqrt{1 + {\omega^{2}\tau^{2}}}}},$where τ represents the thermal time constant of the membrane defined byτ=C_(th)/G_(th).

The temperature variations of the membrane follow the incident radiationpower variations. At low frequencies, i.e. ωτ<<1, the amplitude of thetemperature rise, which defines the signal delivered by the detector, isinversely proportional to G_(th):

$\Theta = {\frac{P_{abs}}{G_{th}}.}$

At high frequencies, i.e. ωτ>>1, the detector signal decreases as theinverse of the modulation frequency. This sensitivity reduction at highfrequencies is all the more marked the larger C_(th):

$\Theta = {\frac{P_{abs}}{\omega\; C_{th}}.}$

The transition between these two regimes is characterized by the thermaltime constant τ.

It results from this analysis that the basic characteristics definingthe performance of the thermal detector are the thermal conductanceG_(th) and the heat capacity C_(th), which it is sought to minimize inorder to optimize the sensitivity of the detector. Consequently, lowthermal conductivity materials are used for the support means, and lowspecific heat materials are used for the absorbent membrane. Inaddition, the membrane generally presents a small thickness.

In order to minimize the thermal conductance of the support means, theair is removed from the space between the membrane 1 and substrate 2(FIGS. 1 and 2) or the space is filled with low thermal conductivitygas. In addition, the arms 3 fixedly secured to the membrane 1 oftenpresent a maximal length, compatible with other constraints. In the caseof simple thermal insulation arms, represented in FIG. 1, the maximallength corresponds substantially to the dimension of the membrane. Adevelopment of this technique consists in fabricating thermal insulationarms folded onto themselves, in the form of a coil, thus presenting alength corresponding to a multiple of the dimension of the membrane. Thedrawback of this technique is that it restricts the surface of theabsorbent membrane and thus restricts the effective surface of thedetector.

The document U.S. Pat. No. 6,144,030 discloses a micro-bolometercomprising thermal insulation arms folded onto themselves and arrangedbetween the membrane and substrate, which enables the effective surfaceto be preserved while lengthening the thermal insulation armsconstituting the support means. However, this construction presentsseveral drawbacks:

-   Mechanical securing of the arms in the form of a coil, cantilevered    by an anchorage point positioned at the end of the coil, requires an    increased thickness of the arms and, thereby, an increase of the    thermal conductance.-   This construction is unsuitable for achieving interferential    cavities, commonly used to optimize radiation absorption. The    interferential cavities presenting the best performances are in fact    usually achieved by placing a reflecting metal layer a few    nanometers thick on the substrate. This reflecting layer, in    conjunction with the membrane, forms a quarter-wave plate centered    on the wavelength to be detected. The arms arranged between the    membrane and the substrate constitute a disturbing element that is    open to criticism. To overcome this difficulty, it is proposed to    position the reflecting layer on the support means, in particular on    the arms. However, infrared-reflecting materials are characterized    by very high thermal conductivities detrimental to a good thermal    insulation.-   This construction leads to an increase of the suspended weight, all    the more so if the option of placing a reflecting layer on the    support means is chosen. This weight increase increases the thermal    time constant and the vulnerability of the detector to mechanical    aggressions, for example shocks and vibrations.-   Finally, this construction, in addition to the means for fixing the    arms to the substrate, requires means for fixing the membrane to the    arms, enabling electrical interconnection of a thermometer. This    results in an increased complexity of the production technology.

Another way of minimizing the thermal conductance consists in reducingthe cross-section of the thermal insulation arms or, more generally, ofthe support means. However, too small cross-sections impair themechanical solidity of the detector and can lead to bending of thesupport means, resulting in rocking of the membrane until it comes intocontact with substrate, thus short-circuiting the thermal insulation.

Rocking can be prevented by adding a mechanical connection that connectstwo adjacent membranes to one another. The drawback of this mechanicalconnection lies in the thermal coupling between the two membranes, whichleads to an impairment of the spatial resolution of the device. Anothermeans of preventing rocking consists in increasing the number ofanchorage points of the support means on the substrate, however thisincreases the thermal conductance.

In general, optimization of thermal radiation detectors involves makinga compromise between the length of the support means on the one hand andthe cross-section of the latter on the other hand, a compromisearbitrated by their mechanical strength.

The document WO 03,011,747 describes a gas sensor comprising a suspendedmembrane connected to a substrate by means of porous silicon bridges andcantilevers. The bridges and cantilevers are arranged in the same(horizontal) plane as the suspended membrane. They are directly formed,in this plane, in the substrate the parts whereof corresponding to thebridges, cantilevers and membrane are rendered porous. A cavity is thenhollowed out under the membrane and under the bridges and cantilevers.

OBJECT OF THE INVENTION

The object of the invention is to remedy these drawbacks and, inparticular, to provide a detector, comprising an absorbent membrane andsupport means, presenting a high thermal insulation capacity whileensuring an enhanced mechanical securing.

According to the invention, this object is achieved by the accompanyingclaims and, more particularly, by the fact that the support meanscomprise at least one alveolate structure arranged in a planesubstantially perpendicular to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will become more clearly apparent from thefollowing description of particular embodiments of the invention, givenas non-restrictive examples only and represented in the accompanyingdrawings, in which:

FIGS. 1 and 2 represent a thermal detector according to the prior artrespectively in top view and in cross-section along the line AA.

FIGS. 3, 4, 6 and 7 represent particular embodiments of the invention.

FIG. 5 illustrates the particular embodiment represented in FIG. 4, incross-section along the line BB.

FIG. 8 shows a network of resistors, enabling the heat losses of adetector according to the invention to be estimated by analogy with anetwork of electrical resistors.

DESCRIPTION OF PARTICULAR EMBODIMENTS

FIGS. 3 to 6 show a thermal electromagnetic radiation detectorcomprising an absorbent membrane 1, fixed in suspension onto a frontface of a substrate 2, in a direction substantially parallel to thesubstrate 2, by support means thermally insulating the membrane 1 fromthe substrate. The support means comprise, as in FIG. 1, two thermalinsulation arms 3, fixedly secured to the membrane 1 and both fixed ontoa thin pillar 4, and two alveolate structures. Each alveolate structureis formed by a wall arranged in a plane substantially perpendicular tothe substrate 2 and, therefore, perpendicular to the membrane 1. Eachalveolate structure presents a plurality of transverse openings. Thethermal insulation arms 3 extend along two opposite sides of themembrane and the length thereof corresponds substantially to thedimension of the membrane. Each of the alveolate structures is arrangedrespectively between one of the two arms 3 and the substrate 2 and is incontact with the corresponding arm 3 via at least one bearing point 5(three bearing points in FIG. 3 and a single bearing point in the otherfigures). The alveolate structure can also be arranged between theabsorbent membrane 1 and the substrate 2, along one edge of theabsorbent membrane 1.

The walls represented in FIGS. 3 to 5 each comprise a plurality ofsuperposed thin layers 6, separated by spacers 7 or 8. The spacersrepresented in FIG. 3 are formed by partitions 7, perpendicular to thesubstrate 2, whereas the spacers represented in FIGS. 4 and 5 are formedby hollow cylinders 8, also arranged perpendicularly to the substrate 2.The alveolate structure can have different shapes: parallelepipedic, asrepresented in FIG. 3, pyramidal, inverted pyramidal, etc. . . . Thealveolate structure can be arranged at different locations under thesuspended structure, along thermal insulation arms 3, as represented inFIGS. 3 and 4, or on the contrary along one side of the membraneperpendicular to the arms 3. Thus, the zone situated under the membrane1 remains free from any disturbing element liable to impair thefunctioning of an interferential cavity between the substrate 2 andmembrane 1, achieved by deposition of a reflecting layer on thesubstrate under the membrane 1 to improve the absorption ratio of thedetector.

The wall represented in FIG. 6 comprises three superposed rows ofarcades 9 formed by thin layers, a first row of arcades 9 being arrangedon the front face of the substrate 2, each arcade of one of the top rowsbeing arranged on the top parts of two adjacent arcades 9 of the bottomrow.

In an alternative embodiment of the invention, represented in FIG. 7,the alveolate structure comprises a porous pad 10, which by naturerepresents an alveolate structure. Different materials can be suitablefor this application, in particular silicon oxides deposited by gel soltechniques, or porous silicon obtained by anodic oxidation in HF mediumof the crystalline silicon and which can be added onto the substrate 2acting as support, prior to construction of the suspended structures.After deposition, or addition of the porous material if this is thecase, the porous layer is delineated by standard photolithography andetching processes to define the pad 10.

Advantageously, the substrate 2 is made of silicon and can compriseintegrated electronic devices enabling the signal coming from thethermometer (not represented) positioned on the membrane and measuringthe temperature rise of the membrane to be read and processed. Thethermometer can for example be a thermistor or a pyroelectric,ferroelectric or thermoelectric sensor. In the particular case ofthermistors, numerous materials can be suitable, in particularsemi-conductors, such as amorphous, polycrystalline or crystallinesilicon or germanium; metal oxides, for example vanadium oxides,manganites; metals with a high temperature coefficient, for exampletitanium-base alloys or Fe—Ni alloys.

The membrane 1 can be formed by the material constituting thethermometer itself or by any other material whose chemistry iscompatible with the material constituting the thermometer, for exampleby silicon oxides, nitrides or any other dielectric semi-conductor.

The thermal insulation arms 3 can be made from at least one of thematerials that constitute the membrane 1. In this case, and if themembrane comprises several suspended layers, the arms 3 extend as acontinuation of at least one of the layers forming the membrane. Toensure thermal insulation, the arms 3 can for example be made of siliconoxide, silicon nitride or amorphous silicon.

In addition to their mechanical securing role, the pillars 4 can alsoperform electrical interconnection between the thermometer electrodes,which can be extended along the thermal insulation arms 3, and theinputs of the electronic signal reading and processing devices which areadvantageously arranged on the substrate 2 or, possibly, on a printedcircuit board located nearby.

The alveolate structures support the membrane 1 mechanically, providingat least one bearing point 5, without however increasing the thermalconductance of the support means. The bearing point or points 5 enablethe cross-section of the arms 3 to be reduced, which enables a higherthermal insulation to be achieved than in the prior art.

An estimation of the thermal insulation capacity of the alveolatestructures enables the advantages of the invention as compared with theprior art to be highlighted, considering a square thermal detector with25 μm sides, corresponding to the most advanced devices currently beingdeveloped for infrared imagery. The ultimate thermal insulation(R_(rad)) of a flat detector is limited by the radiative losses whichdepend both on the surface (S_(D)) of the detector and on its operatingtemperature (T):

${R_{rad} = \frac{1}{4\left( {2S_{D}} \right)\sigma\; T^{3}}},$where σ is the Stefan-Boltzmann constant.

At ambient temperature and for the detector size mentioned, the ultimatethermal insulation is theoretically 160 MK/W. The thermal insulation ofa known detector is lower as it is reduced by the heat losses byconduction in the thermal insulation arms. The usual values arecomprised between 5 MK/W and 30 MK/W.

The thermal resistance of the alveolate structure can be modeled by anetwork of resistances, represented in FIG. 8. This network is formed onthe one hand by resistances R₁ which represent the alveolate structure,and on the other hand by resistances R₂ which correspond to one of thethermal insulation arms 3. An alveolate structure is considered havingfour levels formed by layers of silicon oxide with a width of 1 μm and athickness of 6 nm, separated from one another by vertical partitions thethermal impedance whereof is ignored. The vertical partitions aredistributed uniformly with a pitch of 5 μm along one and the same leveland are arranged staggered from one level to the other. The thermalinsulation arm, with a width of 1 μm, is formed by a 20 nm layer ofsilicon oxide and a 5 nm layer of titanium nitride. The length of thethermal insulation arm is 17 μm and it is made up of four sections, theend of each section bearing on the underlying alveolate structure onfour bearing points 5. For a detector comprising two assemblies eachcomprising a thermal insulation arm and an alveolate structure andtaking the thermal conductivity of the materials into account, we find athermal insulation of 90 MK/W, i.e. three times greater than that of theprior art.

When the number of bearing points 5 is reduced, the thermal insulationincreases slightly. For example, the thermal insulation obtained with asingle bearing point is 98 MK/W, i.e. very close to the result obtainedwith four bearing points.

When the number of the number of levels is reduced, the thermalinsulation is also reduced. For example, the thermal insulation obtainedwith two levels is 72 MK/W. A structure with four levels thereforecorresponds to a good compromise for a detector with a pitch of 25 μm,enabling the thermal insulation to be substantially improved while atthe same time limiting the number of operations required forconstruction thereof.

A method for producing a detector according to the invention comprisesfabrication of the alveolate structure, before the suspended membrane isachieved.

To achieve the alveolate structure represented in FIGS. 4 and 5, formedby superposed thin layers 6 separated by hollow cylinders 8, asacrificial layer, made for example of polyimide, is deposited on thesubstrate 2 or on a thin layer 6, and is annealed. This sacrificiallayer is then etched locally by photolithography making an alignment ofholes with a diameter of 1 μm which pass through the whole thickness ofthe sacrificial layer. Then a dielectric layer with a typical thicknessof 10 nm, covering the sacrificial layer and coating the sides andbottoms of the 1 μm diameter holes, is deposited for example by CVD. Asubsequent step consists in delineating the extent of the dielectriclayer by photolithography and etching to define the cross-section of thewall of the alveolate structure. A first thin layer 6 arranged on hollowcylinders 8 is thus obtained. This sequence of operations is thenrepeated for each additional thin layer 6, taking care to stagger theholes in zig-zagged manner from one level to the other, for example byusing a suitable set of masks. The bearing point 5 on all of the thinlayers is achieved by following substantially the same steps. The finalstep consists in eliminating the sacrificial layers, typically by meansof an oxidizing dry etching.

To achieve the alveolate structure represented in FIG. 6, formed bysuperposed arcades composed of thin layers, a sacrificial layer is alsodeposited and annealed. The material chosen to form the sacrificiallayer is a material presenting an etching rate close to those ofcommonly used photoresists, for example a polyimide. Then a photoresistis deposited in which openings of suitable width opening out onto theunderlying sacrificial layer are delineated, for example by exposurethrough a mask and photographic development. Thermal treatment is thenperformed to make the flanks of the photoresist creep so as to give themthe shape of an arc of a circle. Combined etching of the photoresist andof the sacrificial layer then enables the arc of a circle structures tobe reproduced in the sacrificial layer in known manner. A subsequentstep consists in depositing a dielectric layer, with a thicknesstypically smaller than 10 nm, covering the sacrificial layer and bearingon the underlying substrate at the places where etching of thesacrificial layer was total. Then, the width of the dielectric layer isdelineated, for example to 1 μm, by photolithography and etching, so asto form a row of arcades 9. As in the previous method, the sequence ofoperations is repeated for each row of arcades, each time staggering thearcades one half-period with respect to the underlying arcades.

1. A thermal electromagnetic radiation detector comprising an absorbentmembrane fixed in suspension onto a front face of a substrate, in adirection substantially parallel to the substrate, by support meansthermally insulating the membrane from the substrate, wherein thesupport means comprises at least one alveolate structure arrangedsubstantially perpendicularly to the front face of the substrate and tothe plane of the membrane and in contact with the membrane by a limitednumber of bearing points.
 2. The detector according to claim 1, whereinthe alveolate structure comprises a porous pad.
 3. The detectoraccording to claim 1, wherein the alveo late structure is in contactwith the membrane by a single bearing point.
 4. The detector accordingto claim 1, wherein the alveolate structure is in contact with themembrane by three bearing points.
 5. A thermal electromagnetic radiationdetector comprising an absorbent membrane fixed in suspension onto afront face of a substrate, in a direction substantially parallel to thesubstrate, by support means thermally insulating the membrane from thesubstrate, wherein the support means comprises at least one alveolatestructure arranged substantially perpendicularly to the front face ofthe substrate and to the plane of the membrane, wherein the alveolatestructure is arranged between the absorbent membrane and the substrate,along one edge of the absorbent membrane.
 6. A thermal electromagneticradiation detector comprising an absorbent membrane fixed in suspensiononto a front face of a substrate, in a direction substantially parallelto the substrate, by support means thermally insulating the membranefrom the substrate, wherein the support means comprises at least onealveolate structure arranged substantially perpendicularly to the frontface of the substrate and to the plane of the membrane, wherein thesupport means comprises at least one arm fixedly secured to theabsorbent membrane, each alveolate structure being respectively arrangedbetween the corresponding arm and the substrate.
 7. The detectoraccording to claim 6, wherein the alveolate structure is in contact withan arm by a single bearing point.
 8. A thermal electromagnetic radiationdetector comprising an absorbent membrane fixed in suspension onto afront face of a substrate, in a direction substantially parallel to thesubstrate, by support means thermally insulating the membrane from thesubstrate, wherein the support means comprises at least one alveolatestructure arranged substantially perpendicularly to the front face ofthe substrate and to the plane of the membrane, wherein the alveolatestructure is formed by a wall presenting a plurality of transverseapertures.
 9. The detector according to claim 8, wherein the wallcomprises a plurality of superposed thin layers separated by spacers.10. The detector according to claim 9, wherein the spacers are formed bypartitions perpendicular to the substrate.
 11. The detector according toclaim 9, wherein the spacers are formed by hollow cylinders arrangedperpendicularly to the substrate.
 12. The detector according to claim 8,wherein the wall comprises at least two superposed rows of arcadesformed by thin layers, a first row of arcades being arranged on thefront face of the substrate, an arcade of another row being arranged onthe top parts of two adjacent arcades of a lower row.